In a manufacturing process of a semiconductor device or a FPD (Flat Panel Display), plasma is used to perform a process, such as etching, deposition, oxidation or sputtering, so as to perform a good reaction of a processing gas at a relatively low temperature. Conventionally, plasma generated by a high frequency electric discharge in MHz frequency band has been used in this kind of plasma process. The plasma generated by the high frequency electric discharge is largely divided into capacitively coupled plasma and inductively coupled plasma according to a plasma generation method (in view of an apparatus).
Generally, in an inductively coupled plasma processing apparatus, at least a part (for example, a ceiling) of walls of a processing chamber may have a dielectric window, and a high frequency power is supplied to a coil-shaped RF antenna positioned at an outside of this dielectric window. The processing chamber serves as a depressurizable vacuum chamber, and a target substrate (for example, a semiconductor wafer and a glass substrate) is provided at a central region within the chamber. A processing gas is supplied into a processing space formed between the dielectric window and the substrate. A high frequency AC magnetic field having magnetic force lines is generated around the RF antenna by a high frequency current flowing in the RF antenna. The magnetic force lines of the high frequency AC magnetic field are transmitted to the processing space within the chamber via the dielectric window. As the RF magnetic field of the high frequency AC magnetic field changes with time, an inductive electric field is generated in an azimuth direction within the processing space. Then, electrons accelerated by, this inductive electromagnetic field in the azimuth direction collide with molecules or atoms of the processing gas so as to be ionized. In this process, donut-shaped plasma may be generated.
Since a large processing space is formed within the chamber, the donut-shaped plasma can be diffused efficiently in all directions (particularly, in a radial direction) and a plasma density on the substrate becomes very uniform. However, only with a conventional RF antenna, the plasma density on the substrate is not sufficiently uniform for most plasma processes. In the inductively coupled plasma processing apparatus, it is also one of the important issues to improve uniformity of the plasma density on the substrate since a uniformity/reproducibility and a production yield of a plasma process depend on the uniformity of the plasma density.
Among them, there has been known a technique in which a single RF antenna is used and a passive antenna is provided near the RF antenna (Patent Document 1). This passive antenna is configured as an independent coil that is not supplied with a high frequency power from a high frequency power supply. With respect to a magnetic field of the RF antenna (inductive antenna), intensity of a magnetic field within a loop of the passive antenna is reduced, and intensity of an adjacent magnetic field outside the loop of the passive antenna is increased. Thus, a RF electromagnetic field distribution in a radial direction is changed within a plasma generation region in a chamber.
Further, there has been known a technique in which a RF antenna is divided into a multiple number of circular ring-shaped coils in a radial direction, and these circular ring-shaped coils are connected electrically in parallel to one another in order to increase uniformity of a plasma density distribution in the radial direction (for example, Patent Document 2).    Patent Document 1: Published Japanese Translation of PCT Patent Application No. 2005-534150    Patent Document 2: U.S. Pat. No. 6,164,241
In Patent Document 1, the magnetic field of the RF antenna (inductive antenna) is influenced by the passive antenna, and the RF electromagnetic field distribution in the radial direction is changed within the plasma generation region in the chamber. However, since an operation of the passive antenna is not sufficiently studied and verified, a specific apparatus configuration capable of accurately controlling the plasma density distribution as desired by using the passive antenna is not suggested.
In a recent plasma process, as a substrate becomes larger and a device becomes miniaturized, plasma of a high density and a large diameter is demanded. Therefore, a process on a substrate needs to be more uniformed than before. In this regard, an inductively coupled plasma processing apparatus is configured to generate donut-shaped plasma inside a dielectric window close to a RF antenna and diffuse the donut-shaped plasma in all directions toward the substrate. A diffusion state of the plasma varies depending on an internal pressure of a chamber, and a plasma density distribution on the substrate is easily changed. The plasma density distribution within the donut-shaped plasma may be changed depending on a high frequency power supplied to the RF antenna or a flow rate of a processing gas introduced into the chamber. Therefore, if a magnetic field of the RF antenna (inductive antenna) is not controlled so as to maintain uniformity of a plasma process on the substrate even when a processing condition of a process recipe is changed, it is not possible to satisfy diverse and high-degree of process performances required by a recent plasma processing apparatus.
In the above-described conventional RF antenna division method, a RF current supplied to the RF antenna from a high frequency power supply may flow in a greater amount through an inner coil having a smaller diameter (i.e., smaller impedance), whereas a relatively small amount of RF current may flow through an outer coil having a larger diameter (i.e., larger impedance) within the RF antenna. Accordingly, plasma density within the chamber may be high at a central portion of the chamber in a radial direction while the plasma density may be low at a peripheral portion thereof. Thus, in the second type method, capacitors for adjusting impedance are additionally coupled to the respective coils within the RF antenna so as to adjust a split ratio of the RF current flowing in the respective coils.
In such a case, if a capacitor for adjusting impedance is provided on a return line or an earth line of the high frequency power supply, i.e., on an end of the RF antenna, an electric potential of a coil may become higher than a ground potential, so that an effect (a sputtering effect) causing damage and degradation of the dielectric window by ion attack from the plasma can be suppressed. However, since the coil of the RF antenna is electrically terminated through the capacitor, a length of an equivalent short-circuit resonance line is shortened. As a result, standing wave of electric current having a node in the vicinity of a RF input terminal of the RF antenna may be easily formed (i.e. a wavelength effect may easily occur) in the outer coil having the larger diameter (length). If the wavelength effect occurs, it may be difficult to achieve uniformity of plasma density distribution in a radial direction as well as in a circumferential direction.